This invention relates to a vehicle speed detecting device and more particularly to a device for indicating vehicle speed that is accurate through all anticipated ranges of speed travel.
It is, of course, desirable to provide with many types of vehicles a speed indicator that indicates the speed of travel of the vehicle. Although achieving this goal with land vehicles is relatively simple, it is a much more complicated problem in connection with water vehicles. The reason for this is that there area a wide variety of factors which can effect the accuracy of a water speed indicator in a watercraft.
Recently, it has been discovered that a very effective water speed indicator can be provided by utilizing a source of pressure on the hull of the watercraft or associated with it that senses the dynamic pressure created by the movement of the hull through the water. A pressure transducer of the semiconductor type is employed to convert the water pressure signal into an electrical signal that can be employed to provide a speed indication for the watercraft. A type of system constructed in accordance with this principle is disclosed in the copending application for United States Letters Patent entitled "Speedometer For Marine Vessels", Ser. No. 160,124, filed Feb. 25, 1988 in the name of Kazuhiro Nakahama et al, and assigned to the assignee of this application.
FIG. 1 of the drawings shows generally a system of the type shown in that application wherein a watercraft, indicated generally by the reference numeral 21, has an outboard motor 22 affixed to its transom. The outboard motor 22 includes a drive shaft shaft housing and lower unit 23 in which a forwardly facing pressure sensing opening 24 is formed. A conduit 25 extends from the pressure sensing opening 24 through the hull of the watercraft 21 to a pressure transducer 26 that provides an output signal that is indicative of pressure and a conversion of this pressure signal into a digital signal indicative of speed. An indicator 27 is positioned in the operator's compartment of the watercraft 21 and displays watercraft speed.
FIG. 2 is a graph showing the relationship between pressure P sensed by the sensing device 26 and speed of the watercraft v. It will be seen that at lower speeds and pressures, the pressure speed curve has a relatively sharp incline. At higher speeds, however, the pressure speed relationship is more nearly constant. Basically, the following equation holds between the dynamic pressure P and vessel speed v: EQU v=.sqroot.a.multidot.P+b/P +c.
That is, the lower the vessel speed, the larger of the ratio of vessel speed increment to the dynamic pressure increment as clearly revealed in FIG. 2. Normally, the output from the device is transmitted to an analog to digital converter (ADC) that divides the output from the semiconductor pressure sensor 26 uniformly into a number of steps, such as 256 steps. As a result, as the watercraft speed becomes lower, the resolution of the ADC also becomes lower and the detected speed accuracy is deteriorated. Furthermore, the detection of low pressure levels is difficult due to the varying characteristics which exist at the pressure sensing opening 24 under low speed and pressure conditions.
It is, therefore, an object of this invention to provide an improved, simplified speed detecting device for a watercraft.
It is a further object of this invention to provide a watercraft speed detecting device that is accurate at all speed conditions.